The present invention relates to a display panel having electroluminescent pixels, such as organic electroluminescent pixels.
There has been known an organic electroluminescent (EL) cell wherein fluorescent substance formed on a glass plate or a transparent organic film is applied with electric current to emit light. A plurality of EL cells are disposed as pixels to form an EL display panel.
Referring to FIGS. 9 and 10, a conventional EL display panel has a glass substrate 91 on which a plurality of strips of transparent electrodes 2 is disposed. On the transparent positive electrodes 2, a laminated organic thin layer 21, each layer of which is made of an organic compound, is formed. The organic layer 21 comprises a hole conduction layer 3 formed on the transparent electrodes 2, light emitting layer 4 formed on the hole transport layer 3, and an electron transport layer 5 on the light emitting layer 4. A plurality of metal negative electrodes 6 are disposed on the organic layer 21. The transparent positive electrodes 2 and the negative electrodes 6 are connected to an external power source 7.
The hole transport layer 3 serves to transmit holes from the transparent positive electrode 2 and to block electrons while the electrode transport layer 5 serves to transmit the electrons from the negative electrode 6. When an electron injected through the negative electrode 6 and a hole injected through the transparent positive electrode 2 are bound together, an exciton is generated. The exciton, during the quenching thereof, radiates light which is emitted out through the transparent positive electrode 2 and the glass substrate 91.
The transparent positive electrode 2 is made of such transparent conductive material as a indium-tin oxide (ITO) and other tin oxides having a large work function and capable of transmitting light to the ambience. For the negative electrode 6, aluminum, magnesium, indium, and silver, each having a small work function, are used singularly or as an alloy thereof, namely, aluminum-magnesium alloy and silver-magnesium alloy.
The material for the light emitting layer 4 is for example, 8-hydroxyquinoline-aluminum complex, and for the hole transport layer 3, N'-diphenyl-N, 1'-biphenyl-4, and 4'-diamine (TPD), for example, are preferable.
For the electron transport layer 5, an aluminum complex of 8-hydroxiyquinoline, for example, is used.
Each of the positive electrode and negative electrode layers and organic layers has a very small thickness in the order of several tens to several hundred nanometers. Hence the EL pixel is generally provided with the glass substrate 91 having a relatively large thickness in the order of millimeters as a support on which the layers are attached.
When producing the EL display panel, the transparent positive electrode 2 and the negative electrodes 6 are disposed to extend in perpendicular direction to each other as shown in FIG. 10, thereby forming a matrix. The pixel is formed at each juncture of the positive electrode 2 and the negative electrode 6.
In the above-described EL pixel, the light is radiated to the atmosphere from the luminescent layer 4 through the hole transport layer 3, transparent ITO positive electrode 2, and the glass substrate 91 in a wide range of zero to 180 degrees with respect to the light emitting surface of the light emitting layer 4. However, the light is refracted as the light passes through the boundary surface of each layer. If the refractive index of a material into which the light enters is larger than that of a material from which the light emerges, a light beam having an angle of incidence larger than the critical angle is totally reflected. That is, when an exit angle of emergence of a refracted wave is larger than 90 degrees, the incident light beam is totally reflected.
The relationship between the refractive indices at the boundary surface between two difference mediums is described. It is known from the Snell laws of refraction that when a refractive index n.sub.1 of the emerging medium is larger than a refractive index n.sub.2 of the incident medium, a critical angle e is expressed as follows. EQU .theta.=sin.sup.-1 (n.sub.2 /n.sub.1)
For example, as shown in FIG. 11, when the light emitted from an emitting portion 31 is transmitted through the glass substrate 91 having the refractive index of 1.5 and out into the atmosphere having the refractive index of 1, the critical angle .theta. is calculated as, EQU .theta.=sin.sup.-1 (1/1.5)=41.8.degree.
Hence, although some of the light beams emitted from the light emitting portions 31 are transmitted as shown by arrows L.sub.1 and L.sub.2, the light beams at the incident angle larger than 41.8 is reflected from a boundary surface S as shown by an arrow L.sub.3. Thus the totally reflected light is blocked without being transmitted out into the atmosphere so that the quantity of transmitted light is decreased. As a result, the visually effective transmitting efficiency of the emitted light is decreased.